Performance control system and method for gas discharge lasers

ABSTRACT

A method is provided for determining the status of a gas mixture of a laser system including a gas discharge laser which generates an output beam and has a discharge chamber containing a gas mixture within which energy is supplied to the gas mixture by a power supply via application of a driving voltage to a discharge circuit. A master data set of an output parameter such as any of output beam energy, bandwidth, spectrum width, long axial beam profile, short axial beam profile, beam divergence, energy stability, energy efficiency, width of the discharge, temporal beam coherence, spatial beam coherence, spatial pulse width, amplified spontaneous emission and temporal pulse width versus an input parameter such as driving voltage is generated corresponding to an optimal gas mixture status, preferably after a new fill and typically at the factory, and alternatively following a new fill at the fab. Preferably several master data sets are measured and stored following new fills corresponding to various ages and other conditions of the laser system. The master data set is stored into a memory of the control system. Several more calibration data sets are also preferably generated corresponding to other gas mixture or laser operating conditions. At another time, during a check sub-routine preferably during a subsequent start-up procedure or during each subsequent start-up procedure, a current status data set of the output and input parameters is generated corresponding to the current status of the gas mixture. The current status data set is then compared with the master data set and deviations of the current status and master data sets are noted. A follow-up procedure may be performed, preferably automatically as initiated by the processor, based on the comparison.

PRIORITY

This Application is a continuation application which claims the benefitof priority under 35 U.S.C. 1.53(b) to U.S. patent application Ser. No.09/718,805, filed Nov. 22, 2000 now U.S. Pat. No. 6,330,267, which is acontinuation and claims the benefit of priority under 35 U.S.C. 1.53(b)to U.S. patent application Ser. No. 09/379,034, filed Aug. 23, 1999, nowU.S. Pat. No. 6,212,214, issued Apr. 3, 2001, which claims the benefitof priority to U.S. provisional patent applications No. 60/160,084,filed Oct. 5, 1998, and No. 60/150,683, filed Aug. 5, 1999, saidapplications being incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for determininga status of a laser gas mixture within a discharge chamber of a gasdischarge laser, and more particularly for measuring a parameter of theoutput laser beam versus the applied driving discharge voltage or otheradjustable input parameter and comparing the measured data with a masterset or sets of stored data to determine the status of the laser gasmixture and/or whether any electrical, mechanical or optical problemsexist within the system.

2. Discussion of the Related Art

Gas discharge lasers, e.g., excimer or molecular lasers, are well knownas valuable tools for many industrial applications. There is a greatdesire to have precise control and simultaneous stabilization of manylaser parameters over extended durations of operation, especially withregard to excimer laser applications in many fields includingelectronics and photolithographic processing. The amount of “up time” ofa laser, or time when a laser is in operation and being used forindustrial application, is a key variable in operation costconsiderations. It is desired to be able to successively adjust,sensitively control and carefully stabilize various laser parametersefficiently and simultaneously.

The type and quality of the gas discharge affects many significant laserparameters such as output power, energy stability, efficiency,bandwidth, long and short axial beam profiles, temporal and spatialpulse width, and beam divergence and coherence. The quality of the gasdischarge depends on such factors as the composition of the gas mixturein the discharge chamber, the quality of preionization used, propertiesof the discharge circuit, and profiles of the electrodes used. See R. S.Taylor, Appl. Phys. B41, 1-24 (1986). Decomposition and contamination ofthe gas mixture and the design of the gas exchange system (e.g., flowspeed) also strongly determine the limits of achievable laserparameters. A fast gas exchange between electrodes may be realized byusing a laser discharge chamber design including fast blower gascirculation. Cryogenic and electrostatic equipment and techniques may beused for additional gas purification. See German Patent No. 32 12 928.

Optimum gas mixtures for various gas discharge lasers are generallyknown. A partition of 0.1:1.0:98.9 F₂:Kr:Ne is thought to besubstantially optimal for a KrF-excimer laser, for example, and 0.1:99.9F₂:Ne for an F₂ laser. FIG. 1 shows a plot of laser output power versusF₂ concentration and represents a way of determining what the optimal F₂concentration actually is. As time goes by and the laser is operated,the gas mixture degrades or “ages” continuously resulting in a dilutionof F₂ and a consumption of F₂ via chemical reactions with metal dust. Inthis regard, U.S. Pat. No. 4,977,573 to Bittenson et al., which isassigned to the same assignee as the present application, isincorporated herein by reference. After a parameter changes by a certainamount, such as after a certain amount of time, number of pulses orchange in discharge voltage compensating the gas deterioration tomaintain constant output energy, among others, a replenishment such ashalogen injection (HI) or partial gas replacement (PGR) of a certainamount of the gas mixture or an entirely new fill of the gas mixture isperformed to, as nearly as possible, substantially reinstate theoriginal partition of the gas and optimize laser parameters.

It is desired to be able to prolong the lifetime of the laser gasmixture. It is further desired to have suitable measuring tools thatindicate when and to what extent the laser gas mixture is aged beforeproblems associated with laser parameters varying from optimum as thelaser gas degrades lead to significant reductions in laser outputperformance, processing errors and excessive laser downtime.

A mass spectrometer may be used for precision analysis of thecomposition of the gas mixture. See U.S. Pat. No. 5,090,020 to Bedwell.However, a mass spectrometer is an undesirably hefty and costly piece ofequipment to incorporate into a continuously operating excimer ormolecular laser system. Other ways of monitoring the status of a lasergas mixture include measuring a spectrum width or bandwidth of a laseremission (see U.S. Pat. No. 5,450,436 to Mizoguchi et al.), measuring abeam profile of the laser emission (see U.S. Pat. No. 5,642,374 toWakabayashi et al.), and measuring other characteristics such as thewidth of the discharge or temporal pulse width of the output beamwherein a rough estimate of the status of the gas mixture may be made.See U.S. Pat. No. 5,440,578 to Sandstrom. Another known technique ofmeasuring the age of the laser gas mixture is to count the total numberof laser pulses from the most recent new fill of the discharge chamber.See U.S. Pat. No. 5,646,954 to Das et al.

A number of techniques are known wherein the output beam energy orefficiency is monitored and steps are taken to maintain the output beamat an optimum energy. See U.S. Pat. No. 3,899,750 to Hochuli, U.S. Pat.No. 4,429,392 to Yoshida et al., and U.S. Pat. No. 4,977,573 toBittenson et al. Rare and halogen gas concentrations have also beenmaintained by using a complex series of chemical reactions to determinethe gas mixture concentrations and replenish depleted gases as needed.See U.S. Pat. No. 4,740,982 to Hakuta et al.

The above parameters measured and monitored for determining the statusof the laser gas mixture are each dependent on other parameters inaddition to the gas mixture status, e.g., stabilized output energy,repetition rate, etc. They are based on generally known behaviors oflaser systems and general experience regarding gas mixture aging indischarge chambers. It is desired to have a technique for monitoring thegas mixture status without variations in other parameters affecting theanalysis. It is also desired that properties of the discharge chamber,optics and discharge circuit, among others, be taken into account in agas mixture status monitoring procedure to provide greater completenessand accuracy.

SUMMARY OF THE INVENTION

An effective and sensitive method and apparatus for determining a statusof a gas mixture and its degree of aging or degradation is provided bythe present invention. An internal computer control system determines astatus of a laser gas mixture in a discharge chamber preferably during aspecial check sub-routine. An output beam parameter, e.g., pulse energy,bandwidth, long or short axial beam profile, energy stability, energyefficiency, amplified spontaneous emission (ASE), discharge width, beamdivergence, beam coherence, spatial pulse width or temporal pulse width,versus an input parameter, e.g., driving voltage, are measured andstored by an internal control system. The measured data are thencompared with a master profile or master set of data measured whenoptimal gas mixture conditions exist such as after a new fill of thedischarge chamber with the gas mixture. Preferably, multiple master datasets are measured each corresponding to different operating conditionsof the laser separate from gas mixture status.

A method is provided for determining the status of a gas mixture of agas laser system which generates an output beam having at least onecharacteristic parameter that is measurable, e.g., output pulse energy,bandwidth, long or short axial beam profile, energy stability, energyefficiency, amplified spontaneous emission (ASE), discharge width, beamdivergence, beam coherence, spatial pulse width or temporal pulse width,within the system and has a discharge chamber containing a gas mixturewithin which energy is supplied to the gas mixture by a power supply viaapplication of a driving discharge voltage to electrodes of a dischargecircuit. A master profile or master data set of the output beamparameter is measured versus an input parameter such as driving voltagefor a laser having specified operating conditions preferably includingan optimal gas mixture depending on the operating conditions. Thismaster data set is stored into the memory of the control system of thelaser for which it is desired to measure the status of the gas mixtureat later times. At another time, preferably during a check sub-routinesuch as during a start-up procedure or during each start-up procedure, acurrent status data set of the output beam parameter measured in themaster data set versus the input parameter measured in the master dataset is measured for the gas discharge laser. The current status data setis then compared with the master data set and deviations are noted, suchas in the values and/or derivatives, e.g., slopes, or integrals at datapoints along the curves defined by the data sets.

A small deviation, e.g., of value or slope, between the master andcurrent status data sets will typically indicate that the laser gasmixture has aged or that the F₂ concentration of the gas mixture issomewhat depleted. A large deviation may indicate that a complete newfill is necessary or that serious mechanical, electrical or opticalhardware problems are present in the system. A follow-up procedure isthen preferably performed depending on whether and what type ofdeviation exists between the master and current status data sets.

A current data set may be measured after a new fill, and the data setcompared with the master data set obtained after a previous new fill.When deviations are present between the master and current data setsjust after a new fill, assuming operating conditions haven't otherwisebeen changed, then hardware problems are suspected, as aging of the gasmixture will have not yet occurred.

In the preferred embodiment, one or more calibration data sets areinitially measured and stored corresponding to different gas mixturestatuses and/or operating conditions of the laser. A processor comparesa current status data set which is deviated from the master data setwith one or more of the calibration data sets to find the calibrationdata set which is most similarly deviated from the master data set. Theprocessor then determines, from stored gas mixture data regarding thesimilarly deviated calibration data set, what type of follow-upprocedure should be performed. For example, a follow-up procedure mayinclude replenishment of a certain amount of a molecule including anactive halide species such as F₂, or HCl, an active rare gas or acombination of a molecular halide and an active rare gas.

A gas discharge laser system is also provided. The laser system includesa discharge chamber containing a laser gas mixture and a resonator forgenerating a laser beam. A power supply circuit delivers energy to thegas mixture by providing a driving voltage to a discharge circuit thatultimately produces a potential difference across electrodes in thedischarge chamber. The system further includes means for measuring aninput parameter, e.g., the driving voltage, and means for measuring anoutput beam parameter which varies with the status of the gas mixture,such as one of those mentioned above, e.g., the output power of thelaser beam. A processor then receives the input and output parametermeasurements as a current status data set. The current status data setis compared with a master data set that represents a substantiallyoptimal gas mixture and a laser operation status. The master data setused may differ between laser systems of varying ages or other operatingconditions such as repetition rate. The current status data set may alsobe compared with one or more calibration data sets, such as weremeasured, e.g., following one or more halogen replenishment procedures,during an initial start-up procedure, during one or more subsequentstart-up procedures, after subsequent new fills or under other thanoptimal gas mixture conditions. Follow-up procedures may be performeddepending on the types of deviations, such as values, derivatives orintegrals at points along curves defined by the data sets, noted fromthe comparison.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of laser output energy versus F₂ partial pressure fora KrF-excimer laser showing that a maximum in output energy exists at adeterminable F₂ partial pressure.

FIG. 2 is a graph showing the qualitative dependence of laser outputenergy on driving voltage of a discharge chamber of a gas dischargelaser.

FIG. 3a shows how the graph of laser output energy versus drivingvoltage of FIG. 2 shifts downward with the age of the laser gas mixture.

FIG. 3b shows how the slope of the laser output energy versus drivingvoltage curve of FIG. 2 depends on the F₂ concentration within the lasergas mixture.

FIG. 4a is a flow chart showing a step-by-step procedure for monitoringthe status of the laser gas mixture and other components of the lasersystem according to the present invention.

FIG. 4b is a flow chart showing a step-by-step procedure for monitoringthe status of the laser gas mixture and other components of the lasersystem, including comparison of a current status data set with one ormore calibration data sets, according to the present invention.

FIG. 5 shows a setup of a laser system arrangement according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the present invention, an output parameter and an inputparameter are measured under optimal gas laser conditions and undervarious other conditions of the gas mixture and laser system toestablish at least a master data set and preferably one or morecalibration data sets. During subsequent operation of the laser, thesame parameters are measured to establish a current data set associatedwith current gas mixture and laser operating conditions. The currentdata set may be measured using a same or a different laser than themaster data set. One or more calibration data sets may also be measuredand stored using a same or a different laser as the current data set.The master and calibration data sets may also be measured using a sameor a different laser. The current data set is compared to the masterdata set, and also preferably at least one of the one or morecalibration data sets particularly when deviations are found between themaster and current data sets. In a preferred embodiment of the presentinvention as described below and in the accompanying drawings, theoutput beam parameter is pulse energy and the input parameter is drivingvoltage. Other parameters may be used as the measured output beamparameter such as bandwidth or spectrum width, long or short axial beamprofiles, beam divergence, temporal or spatial beam coherence, energystability, energy efficiency, the width of the discharge and temporal orspatial pulse width or another parameter which varies with gas mixturestatus of a gas laser such as an excimer or molecular laser. The inputparameter may be another parameter which varies as the laser systemoperates and/or statically ages.

FIG. 2 shows a qualitative plot of the output energy of a gas laser as afunction of driving voltage of a driving circuit of the laser. Thedriving voltage measured is preferably that which is applied to thedischarge circuit from a power supply of the laser. The power supplytypically charges a charging capacitor that is connected to a step-uptransformer. The connection is switch controlled. A voltage from thestep-up transformer is applied to a set of electrodes in the dischargechamber. Another voltage may be alternatively measured such as thevoltage across the electrodes or the voltage of the charging or peakingcapacitors.

In this way, energy is delivered to the laser gas to excite moleculeswithin it to higher energy states, from which deexcitation of themolecules is responsible for lasing action. For some lasers, thisvoltage is applied to a pump medium, rather than the laser active gasitself, but a driving voltage (or current) is nonetheless present innearly every known gas laser system. As shown, the slope of the curve oflaser output energy versus driving voltage of FIG. 2 is everywherepositive and decreasing with voltage. This graph has been generatedusing data values measured experimentally for a wide variety of gasdischarge lasers. That is, the graph shown in FIG. 2 generally shows acharacteristic which is common to gas discharge lasers, includingexcimer and molecular lasers such as KrF, ArF, F₂, XeCl, XeF, and KrCllasers.

The graph of FIG. 2 represents a data set may stored in an internalcomputer control system (hereinafter “processor”) of a laser system. Thecomputer receives data representing values preferably of the drivingvoltage applied to the main discharge circuit from a power supply, whileat the same time receiving data preferably of the output energy, orpower, of the laser beam from an energy or power meter. Several datapoints of the output energy versus the driving voltage are measured andstored. Preferably a graph such as that shown in FIG. 2 is generated andproduced by the processor. The data is taken and the graph is generatedpreferably during a special subroutine performed during a start-upsub-routine of the gas laser system wherein it is desired to check thequality of the laser gas mixture within the discharge chamber.

In fact, the relationship between driving voltage and laser outputenergy has a unique dependence on specific characteristics of the lasergas mixture. Inspection of the graph generated during the start-upprocedure indicates quantities relating to the laser gas mixture such asthe relative apportionment of individual gases, total pressure, and theaging and degradation of the gas mixture. It follows that a comparisonof two graphs generated at different times, such as during two differentstart-up procedures, reveal differences in the status of the laser gasmixtures at the two different times when the graphs were generated.Hardware and other discrepancies of the system are also discerniblebetween the two setups at the two different times by comparing the twographs.

A master data set is measured preferably using a laser system which isoptimally configured and has not yet aged. That is, a new fill of alaser gas mixture is performed preferably just prior to measuring thedata of the master data set. At this time, the laser gas mixture has notyet aged and the apportionment of each of the gases, or the percentagesof each gas that comprises the laser gas mixture, is optimal. Moreover,all of the hardware of the system including the electronics and theoptics, as well as the optical alignment of each optical module of thesystem, is optimal when the master data set which may be represented bya master graph generated from the master data set is measured. Thismaster data set, after all of the above are verified, is then stored asa master data set to be compared to each subsequently measured data set.The master data set is preferably measured using data taken after aninitial new fill and warming up of the laser gas mixture to equilibriumtemperature of operation. Measuring the master data set, which ispreferably followed by generating a master graph from the master dataset, is followed by communicating and storing the master data set into amemory accessible by the processor, preferably automatically. The masterdata set may typically be measured at the factory after manufacture of anew laser. The master data set may be generated with the same laser asor with a different laser than it is desired to be able to learn the gasmixture status of at a later time.

More than one master data set may be generated using a laser or lasersof various ages, wherein one is preferably generated using a new laser.As a gas laser itself ages, data sets measured when the gas mixture andother system components are otherwise optimal will typically vary. Thus,it is preferred to have more than one master data set, each master dataset corresponding to a different age range of the laser itself. Forexample, three master data sets may be measured, and master graphspreferably generated therefrom, and used in the present invention. Thefirst master data set corresponds to an optimal condition of a laserthat is, e.g., less than 10⁹ laser pulses old. The second corresponds toan optimal condition of a laser that is, e.g., from 1-2.5×10⁹ laserpulses old. The third corresponds to an optimal condition of a laserthat is, e.g., older than 2.5×10⁹ pulses old.

Preferably, several “calibration” data sets are also measured and storedin the memory accessible by the processor. Each calibration data setcorresponds to a different status of the gas mixture. A bank of datasets corresponding to gas mixtures of various aging states and includingvarious F₂ concentrations are then available for the processor tocompare with subsequently generated data sets corresponding to gasmixtures that are determined to be deviated from optimal, but have anunknown precise status. Calibration data sets may be measured and storedcorresponding to other laser system conditions that practitioners knowand learn develop with time, pulse count or other parameter whichevolves as the laser is operated and/or static or off or in anintermediate mode.

In this way, the processor analyzes many values related to severalpreviously measured data sets of preferably output beam energy versusdriving voltage, and preferably generating graphs or curves from thedata, and compares them to a current data set to determine the currentstatus of the gas mixture. Electrical and mechanical hardware andoptical alignment and degradation problems may also arise from time totime and can be noticed during the check sub-routine described above,realizing another advantage of the present invention.

It is, however, the typical and systematic aging and degradation of thelaser gas mixture which is uniquely observable via comparison betweenthe master data set and/or other previously measured and storedcalibration data sets and the current data set measured at someparticular time after measuring the master data set and the othergraphs. The magnitude and slope variances, in particular, between curvesor graphs representing the master data set and the current data set,e.g., indicate important characteristics of the laser gas mixture. Onesuch characteristic is the degree to which the gas mixture is generallyaged and degraded at the time the second data set is measured. Anothercharacteristic is the reduction of specific gases, such as, e.g., F₂,within the mixture from the time of the most recent new fill until thetime the second data set is generated.

The magnitudes, slopes, etc. of data sets of the output beam parameter,e.g., energy, of the laser for a given value of the input parameter,e.g., driving voltage, depend on the aged status, or the age, of thelaser gas mixture, and are measured preferably in time or pulse countfrom the most recent new fill, but may reference another eventsignificant to the laser system. FIG. 3a qualitatively shows how theoutput energy magnitude decreases as the laser gas mixture ages withtime. Three curves are shown in FIG. 3a. The topmost curve is a mastercurve generated using a new or nearly new laser and data takenimmediately or nearly immediately after a new fill, such as one millionpulses afterwards. The middle curve was generated using data takensometime, e.g., two days or 50 million pulses, after a new fill. Thebottom curve was generated using data taken sometime even later, e.g.,five days or 100 million pulses after a new fill.

It is observed from FIG. 3a that as the laser gas mixture ages, thecurve shifts downward at an expected rate. After more than one new fill,for instance, it can be estimated based on experiences followingprevious new fills how much the laser gas mixture has aged. As the gasmixture ages, many changes take place including reduction of halogenconcentration, and build-up of contaminants such as, e.g., CF₄, anddust. This information is important because appropriate follow-upmeasures can be taken to ensure the reliability of the output beam,which may be used in precise industrial applications.

The slope of the curve of output energy of the gas laser versus drivingvoltage also depends on the percentage concentration of F₂ within thelaser gas mixture. FIG. 3b qualitatively shows how this slope decreasesas the percentage concentration of F₂ within the laser gas mixturedecreases. The curves of FIG. 3b differ from those of FIG. 3a becauseonly F₂ concentration is varied between the curves of FIG. 3b, while, asmentioned above, other factors, such as contaminant and dust build-upthat occur as the laser gas mixture ages, influence the shapes of curvesof FIG. 3a.

Three curves are shown in FIG. 3b. The topmost curve is a master curvegenerated using the master data set from a laser having a first F₂concentration. The middle curve was generated using data from a laserhaving a second F₂ concentration lower than the first concentration. Thebottom curve was generated using data from a laser having a third F₂concentration lower than either of the first or second concentrations.It is observed from FIG. 3b that as the percentage F₂ concentrationdecreases, the slope of the curve decreases at an expected rate. Aftermore than one new fill, for instance, it can be estimated based onexperiences following previous new fills how much the F₂ concentrationhas decreased. This information is important because appropriatefollow-up measures such as reinstating the optimal percentage F₂concentration can be taken to ensure the reliability of the output beam,which may be used in precise industrial applications. Comparison ofcurves such as those of FIG. 3b may also yield characteristic signs ofpercentage concentration reduction with time of other gases of the lasergas mixture.

In summary with respect to FIGS. 3a and 3 b, aging of the laser gasmixture is observed from a drop of the laser output energy versusdriving voltage at a given voltage, revealed by a shift downward of thecurve as shown in FIG. 3a. A lack of certain gas components, such as F₂,can be observed from a decrease in the slope of the laser output energyversus driving voltage curve as shown in FIG. 3b. Comparing a mastergraph generated using a master data set measured immediately or nearlyimmediately after a new fill of a laser with a second graph generatedusing data measured during a check sub-routine of the laser system suchas during a start-up procedure for the laser, and analyzing deviationsbetween the two graphs over an extended range of voltages serves as asensitive technique which reveals deviations of properties of the lasergas mixture over time. The same may be done with other input parametersthan voltage which affect significant output parameters such as pulseenergy, and with other output parameters which vary as the gas mixturedeteriorates.

Moreover, comparison of a graph generated using data taken during asub-routine some time after the most recent new fill and a master graphusing data taken immediately after a new fill of a new laser with anoptimal hardware and optical alignment configuration may or may notyield deviations. If no deviations are found, then the gas mixture andsystem hardware are determined to be satisfactory under the sub-routineof the present invention. Any deviations which are observed may havebeen observed in a previous comparison, such that conclusions can bedrawn as to whether and what type of follow-up procedures should beperformed and/or any follow-up procedures may then be performed eitherautomatically or as initiated by a user, quickly and efficiently, andfrom experience. For example, a gas handling procedure can be performedto restore the gas mixture to its original starting condition either byreplenishment, gas injections of one or more gases or replacement ofgases or a new fill or otherwise by comparing the measured data setsand/or graphs generated from the data sets, observing any deviations andresponding from experience and/or otherwise acquired knowledge of whatthe observed deviations are indicative of.

Preferably, output energy versus driving voltage curves are measuredsuch as may be represented by a graph or table generated by theprocessor representing the current status of the gas mixture. Instead ofa graph or table, another database configuration may be used. Theprocessor then compares the current data set, graph or table to a storeddata set, graph or table according to the flow chart shown in FIG. 4a,wherein a graph is used as an example. Generally, the processor analyzesany deviations between the current status and optimal or other known gasmixture statuses to determine what type of deviation is present and whatfollow-up procedure should then be performed, after comparing thecurrent graph with the master graph and/or any of the processor's storedbank of graphs. A preferred procedure is more specifically set forth inthe flow chart of FIG. 4a.

The exemplary procedure shown in FIG. 4a begins with a system power-onsignal initiated by a user or otherwise begins a warm-up period whichculminates in the emission of an output beam from the laser chamber andoptical arrangement of the laser system (step S1). It is next determinedwhether a new fill is requested (step S2). If yes, then a new fill isperformed (step S3). After the new fill is performed in step S3, or ifno new fill is requested, then measurements are taken, after which agraph may be generated, and measurements are of preferably output beamenergy versus driving voltage (step S4). Next, a comparison is madebetween the data measured in step S4, which may be in the form of acurrent graph generated in step S4, and one or more stored data sets,tables or graphs including at least the master data set, table or graph(step S5). In a preferred embodiment, the present age of the laser isfirst checked. Then the master data set, table or graph, of preferably agroup of stored master data sets, tables or graphs, corresponding to theage range that includes the present age of the laser is selected to beused as the master data set, table or graph in step S5.

If the current data set, table or graph and the master data set, tableor graph are determined to be identical, or nearly identical withinspecified tolerances (step S6), then the laser is ready to be used inindustrial applications such as, e.g., photolithography (step S10), andsteps S7-S9 and S11-S13 are not necessary. Thus, the procedure of thepreferred embodiment of the present invention shown in FIG. 4a isfinished for this run.

If the master and current data sets, tables or graphs are not identical,then it is determined whether large or small deviations exist (step S7).If a small deviation or deviations is/are observed, or deviations areobserved which are outside of specified tolerances, then preferably auser is notified via a message (step S8), e.g., on a computer display,or an audio or visual alarm signal. A follow-up procedure may beperformed either by user input, e.g., in response to receiving themessage or signal, or automatically as initiated by the processor usinga preprogrammed software routine (step S9). Such follow-up measures ashalogen injection, partial gas replacement, gas replenishment and totalpressure correction (e.g., by adjusting the gas temperature) may beperformed. Consequent to the follow-up measures, the gas mixture ispreferably returned to a state which deviates minimally from itsoriginal starting condition, such that substantially optimal lasersystem performance is again enabled, and industrial application of thelaser system can go forward (step S10).

If a strong deviation or deviations are determined to be present, ordeviations are observed which are far outside the specified tolerancesdescribed above and may be outside a second set of tolerances (step S7),then it is determined whether a new fill has just been performed or not(step S11). If a new fill has not just been performed, then a follow-upnew fill is now performed (step S3).

After the follow-up new fill is performed, a data set is measured, and agraph or table may be generated, of preferably output energy versusdriving voltage (step S4) and the procedure previously described isfollowed. If a new fill was just performed, and the current data set wasmeasured immediately after the most recent new fill, then it is assumedthat the gas mixture is optimal. In this case where the current data setwas measured immediately after the most recent new fill, such that thegas mixture is assumed to be optimal, it is assumed that any deviationis not as a result of an aged gas mixture.

The fact that an electrical, mechanical or optical hardware problemexists somewhere in the system is thus suggested by the presence of astrong deviation (step S12). Some possible problems include trouble withthe gas delivery system, optics modules of the system such as opticalwindows or line narrowing and/or tuning elements, the laser dischargechamber itself, electrical components such as the power supply and thebeam path purging system. Some gas delivery problems include leakage,contamination of the gas cylinders, fluorine partial pressure changesfrom cylinder to cylinder, and contamination from the walls of nearlyempty cylinders. A user is then preferably prompted to follow clearinstructions to locate the problem and possibly call for service (stepS13). These serious problems indicated by large deviations observed whencomparing the current data set, table or graph with the master data set,table or graph are easily distinguished from gas aging because the largedeviations of the current graph from the master graph are typically farstronger than those due to aging and are not eliminated by performing anew fill.

The method of FIG. 4b varies from that of FIG. 4a in that step 8 of FIG.4a is replaced by steps 8 a and 8 b in FIG. 4b. In accord with themethod of FIG. 4b, a bank of calibration data sets (not shown) ispreferably stored in the memory of the processor sometime before thecurrent status is measured. Typically, the calibration data sets aremeasured at the factory, defining a response of the laser system (or alike laser system) when operating with different gas mixtures of thetypes associated with common aging. Following step S7, after it has beendetermined that a small deviation or small deviations exist between thecurrent status data set, table or graph and the master data set, tableor graph, a comparison of the current status data set, table or graphwith one or more or all of the calibration data sets, tables or graphsis performed (step S8A) to find one or more calibration data sets whichare similarly deviated from the master data set. The processor thendetermines, from stored gas mixture data regarding the one or moresimilarly deviated calibration data sets, what type of follow-upprocedure will be performed that will likely return the gas mixture to amore desirable status. For example, a follow-up procedure may includegas injection, partial replacement or other replenishment of a certainamount of F₂, HCl, an active rare gas or a combination thereof, or totalpressure correction (e.g., by adjusting the gas temperature or totalamount of gas in the discharge chamber). The processor may then initiatethe follow-up procedure such that the follow-up procedure isautomatically initiated and/or performed, with or without the assistanceof a user.

Using the preferred method of the present invention, it is possible todetermine the gas quality and partition composition. It is also easy toschedule times for new fills and gas replenishing follow-up procedures.Moreover, the method is very sensitive such that F₂ concentrations downto at least 3×10⁻⁴ absolute are resolvable. That is, deviations of 0.3percent or possibly less are detectable using the sensitive method ofthe present invention. The method is also efficiently performed by theprocessor using a special checking sub-routine not requiring additionalhardware components to be added to an existing system, makingimplementation of the advantages of the present invention realizable atlow cost. Further, the processor may operate the gas handling system insuch a manner that the actual laser performance is automaticallyreadjusted to the level of the master data set, thus principallyenabling a system having a laser gas mixture with infinite lifetime.

A preferred arrangement of a gas discharge laser system having internallaser parameter control is shown in FIG. 5. The system includes adischarge chamber 1 containing the gas mixture and comprising adischarge circuit including electrodes 9 for driving a discharge throughthe gas mixture. A resonator defines an optical path through the lasertube 1 including, e.g., a high reflectivity reflection grating, highlyreflecting back surface of a prism or a rear mirror 2, and an outputcoupling beam splitter or front mirror 3. The outcoupled portion of thebeam impinges upon a beam splitter 4 such that a portion of theoutcoupled beam is reflected to an internal energy monitor 5. Aninternal computer control system 6, or processor, receives data from theinternal energy monitor 5.

The laser system further includes a power supply 7 for delivering energyto the gas mixture via applying a driving voltage to the electrodes 9 ordischarge circuit of the discharge chamber 1. Values of the drivingvoltage are preferably received by the internal computer control system6 corresponding preferably to detected values of output beam energy asmeasured by the internal energy monitor 5 and also received by thecomputer control system 6. The computer control system 6 preferably maystore in a memory and retrieve from the memory the master andcalibration data sets described above. The control system 6 may thengenerate a table or graph of any of the data sets or store the data setsin any form including a database format using at least a range orportion of the data sets. The control system 6 preferably performs manyother functions as described above. A gas compartment 8 is connectedwith the discharge chamber 1 via gas lines and is flow controlled eitherby the internal computer control system 6 or manually. That portion ofthe outcoupled beam which is not reflected at the beam splitter 4 is theoutput beam 10 of the system.

The specific embodiments described in the specification, drawings,summary of the invention and abstract of the disclosure are not intendedto limit the scope of any of the claims, but are only meant to provideillustrative examples of the invention to which the claims are drawn.The scope of the present invention is understood to be encompassed bythe language of the claims, and structural and functional equivalentsthereof.

What is claimed is:
 1. A method for controlling a status of a laser gasmixture of an excimer or molecular fluorine gas discharge laser systemwhich generates an output beam and includes a discharge chamber filledwith the gas mixture within which energy is delivered by a power supplycircuit to components of the gas mixture, comprising the steps of:operating the laser to generate the output beam; measuring a value of afirst parameter of the laser system which varies substantially with astatus of the gas mixture including a concentration of a halogencomponent thereof and which also tends to vary substantially with one ormore other conditions of the laser system; determining a correspondingvalue of a second parameter of the laser system; and replenishing ahalogen component of the gas mixture based on a known correspondencebetween (i) a relationship between the measured value of the firstparameter and the determined corresponding value of the second lasersystem parameter, wherein the relationship varies substantially with thestatus of the gas mixture and does not tend to vary substantially withany of the one or more other conditions of the laser system which tendto vary the first parameter; and (ii) the status of the laser gasmixture.
 2. The method of claim 1, wherein the first parametercorresponds to an energy of the output beam.
 3. The method of claim 2,wherein the second laser system parameter corresponds to an energydelivered to the gas mixture by the power supply circuit.
 4. The methodof claim 1, wherein the relationship between the measured value of thefirst parameter and the determined value of the second parameterincludes a slope of a plot of one of the values of the first and secondparameters versus the other.
 5. The method of claim 1, wherein therelationship between the measured first parameter and the second lasersystem parameter includes a ratio of a quantity relating to one of thevalues of the first and second parameters to a quantity relating to theother of the first and second parameters.
 6. A method for controlling astatus of a laser gas mixture of an excimer or molecular fluorine gasdischarge laser system which generates an output beam and includes adischarge chamber filled with the gas mixture within which energy isdelivered by a power supply circuit to components of the gas mixture,comprising the steps of: operating the laser to generate the outputbeam; taking at least a first measurement and a second measurement of afirst parameter of the laser system to obtain a first measured value anda second measured value, respectively, wherein the first parametervaries substantially with a status of the gas mixture including aconcentration of a halogen component thereof and tends to varysubstantially with one or more other conditions of the laser system;determining at least a corresponding first and second values,respectively, of a second parameter of the laser system; andreplenishing a halogen component of the gas mixture based on a knowncorrespondence between (i) a relationship between (a) a relationshipbetween the first measured value and the second measured value of thefirst parameter; and (b) a corresponding relationship between the firstvalue and the second value of said second laser system parameter,wherein said relationship between (a) and (b) varies substantially withthe status of the gas mixture and does not tend to vary substantiallywith any of the one or more other conditions of the laser system whichtend to vary the first parameter; and (ii) the status of the laser gasmixture.
 7. The method of claim 6, wherein the first value and thesecond value of the second laser system parameter correspond to valuesof the second laser system parameter when the measurements were taken ofthe first and second measured values of the first parameter.
 8. Themethod of claim 6, wherein the relationship between the first measuredvalue and the second measured value corresponds to a difference betweenthe first measured value and the second measured value.
 9. The method ofclaim 8, wherein the relationship between the first value and the secondvalue of the second laser system parameter corresponds to a differencebetween the first value and the second value of the second laser systemparameter.
 10. The method of claim 6, wherein the relationship betweenthe first value and the second value of the second laser systemparameter corresponds to a difference between the first value and thesecond value of the second laser system parameter.
 11. The method ofclaim 6, wherein the first parameter corresponds to an energy of theoutput beam.
 12. The method of claim 11, wherein the second laser systemparameter corresponds to an energy delivered to the gas mixture by thepower supply circuit.
 13. A method for controlling a status of a lasergas mixture of an excimer or molecular fluorine gas discharge lasersystem which generates an output beam and includes a discharge chamberfilled with the gas mixture within which energy is delivered by a powersupply circuit to components of the gas mixture, comprising the stepsof: operating the laser system to generate the output beam; obtaining atleast first and second different values of a first parameter of thelaser system corresponding to first and second different values,respectively, of a second parameter of the laser system, wherein thefirst parameter varies substantially with a status of the gas mixtureincluding a concentration of a halogen component thereof and tends tovary substantially with one or more other conditions of the lasersystem; replenishing a halogen component of the gas mixture based on aknown correspondence between (i) a relationship between (a) arelationship between said first and second different values of saidfirst laser system parameter; and (b) a corresponding relationshipbetween said first and second different values of said second lasersystem parameter, wherein said relationship between (a) and (b) variessubstantially with the status of the gas mixture and does not tend tovary substantially with any of the one or more other conditions of thelaser system which tend to vary the first parameter; and (ii) the statusof the laser gas mixture.
 14. The method of claim 13, wherein the firstvalue and the second value of the second laser system parametercorrespond to values of the second laser system parameter when the firstand second values of the first laser system parameter were obtained. 15.The method of claim 13, wherein the relationship between the first andsecond values of the first laser system parameter corresponds to adifference between the first and second values of the first laser systemparameter.
 16. The method of claim 15, wherein the relationship betweenthe first value and the second value of the second laser systemparameter corresponds to a difference between the first value and thesecond value of the second laser system parameter.
 17. The method ofclaim 13, wherein the relationship between the first value and thesecond value of the second laser system parameter corresponds to adifference between the first value and the second value of the secondlaser system parameter.
 18. The method of claim 13, wherein at least oneof the first and second laser system parameters corresponds to an energydelivered to the gas mixture by the power supply circuit.
 19. The methodof claim 18, wherein the other of the first and second laser systemparameters corresponds to a number of pulses of the output beam.
 20. Themethod of claim 18, wherein the other of the first and second lasersystem parameters corresponds to a time of operation of the lasersystem.
 21. The method of claim 18, wherein the other of the first andsecond laser system parameters corresponds to an energy of the outputbeam.
 22. The method of claim 13, wherein at least one of the first andsecond parameters corresponds to a number of pulses of the output beam.23. The method of claim 13, wherein at least one of the first and secondlaser system parameters corresponds to a time of operation of the lasersystem.
 24. A method for controlling a status of a laser gas mixture ofan excimer or molecular fluorine gas discharge laser system whichgenerates an output beam and includes a discharge chamber filled withthe gas mixture within which energy is delivered by a power supplycircuit to components of the gas mixture, comprising the steps of:operating the laser system to generate the output beam; obtaining afirst value of a first parameter of the laser system corresponding to asecond value of a second parameter of the laser system, wherein thefirst parameter varies substantially with a status of the gas mixtureincluding a concentration of a halogen component thereof and tends tovary substantially with one or more other conditions of the lasersystem; replenishing a halogen component of the gas mixture based on acomparison of: (a) a relationship between said first value of said firstparameter of the laser system and said second value of said secondparameter of the laser system, wherein said relationship between saidfirst value of said first parameter and said second value of said secondparameter varies substantially with the status of the gas mixture anddoes not tend to vary substantially with any of the one or more otherconditions of the laser system which tend to vary the first parameter;and (b) a known correspondence between (i) said relationship betweenfirst value of said first parameter of the laser system and said secondvalue of said second parameter of the laser system; and (ii) the statusof the laser gas mixture.
 25. A method for controlling a status of alaser gas mixture of an excimer or molecular fluorine gas dischargelaser system which generates an output beam and includes a dischargechamber filled with the gas mixture within which energy is delivered bya power supply circuit to components of the gas mixture, comprising thesteps of: operating the laser system to generate the output beam;obtaining a first value of a first parameter of the laser systemcorresponding to a second value of a second parameter of the lasersystem, wherein the first parameter varies substantially with a statusof the gas mixture including a concentration of a halogen componentthereof and tends to vary substantially with one or more otherconditions of the laser system; replenishing a halogen component of thegas mixture based on a known correspondence between (i) a relationshipbetween said first value of said first parameter of the laser system andsaid second value of said second parameter of the laser system, whereinsaid relationship between varies substantially with the status of thegas mixture and does not tend to vary substantially with any of the oneor more other conditions of the laser system which tend to vary thefirst parameter; and (ii) the status of the laser gas mixture.
 26. Themethod of claim 25, wherein at least one of the first and second lasersystem parameters corresponds to an energy delivered to the gas mixtureby the power supply circuit.
 27. The method of claim 26, wherein theother of the first and second laser system parameters corresponds to anumber of pulses of the output beam.
 28. The method of claim 26, whereinthe other of the first and second laser system parameters corresponds toa time of operation of the laser system.
 29. The method of claim 26,wherein the other of the first and second laser system parametercorresponds to an energy of the output beam.
 30. The method of claim 25,wherein at least one of the first and second parameters corresponds to anumber of pulses of the output beam.
 31. The method of claim 25, whereinat least one of the first and second laser system parameters correspondsto a time of operation of the laser system.
 32. The method of claim 25,wherein at least one of the first and second laser system parameterscorresponds to an energy of the output beam.